The present invention relates specifically to a substantial improvement to the disclosure of U.S. Patent application No. 661,579, filed Feb. 26, 1976 by the same applicant as the present application and relates generally to a method and apparatus for forming a plurality of identical images at equal spacing on the circumference of a photoconductive member such as a drum or endless belt.
In known electrophotographic or electrostatographic copying apparatus, a photoconductive drum is imaged, and the image is developed and transferred during one rotation of the drum. Even if the length of an original document for reproduction is less then 1/2 the circumference of the drum, only one copy will be produced for each rotation of the drum. It is also known in the art to form an electrostatic image on a photoconductive drum, and then develop and transfer the image a plurality of times to copy sheets. In the present state of the art, an electrostatic image may typically be developed and transferred up to 30 times. Even if this process is applied to multiple copy applications, only one copy may be produced for each rotation of the drum regardless of the size of the image.
It is also known in the art in applications in which two or more images of a small original document can be spaced on the circumference of a drum to rotate the drum during one scanning operation to form a first image, stop the drum rotation while the scanning system resets, and again rotate the drum for another scanning operation. Whereas this method increases the copying efficiency by producing more than one copy for each rotation of the drum after all of the images have been formed on the drum using the multiple development and transfer method described above, the mechanism for precisely starting and stopping the drum in dependence on the number of images to be produced is necessarily complex and costly.
In another known method of producing a plurality of images on the drum surface, the drum is rotated continuously and the scanning mechanism is reset at high speed between scanning operations, the speed of various members of the scanning mechanism exceeding 1000mm/sec. during such an operation. The power required for driving a scanning mechanism at this speed is excessive, and the high rates of acceleration and deceleration cause scanning lamps and precision optical components to fail rapidly.
Applicant's prior U.S. Patent application discloses a method of overcoming these problems by forming a plurality of images of the same original document at equal spacing on the circumference of a photoconductive drum or the like. The images are developed a number of times, and the resulting toner images are transferred to copy sheets after each developing step. The method comprises forming an image on one section of the drum and then rotating the drum by a plurality of image spaces before forming an image on another section. The number of image spaces skipped is determined by the number of images to be formed in such a manner that the images will be formed only once during a predetermined number of rotations of the drum.
This method is especially advantageous where large number of copies are to be made and is capable of increasing the copying speed to more than double the speed of normal copying.
However, this prior method is not advantageous where the number of copies to be made is between about 3 and 10. This is because the prior method forms the maximum number of electrostatic images on the drum before development and transfer, and where a small number of copies is required not all of the images are necessary. In addition, the relatively large number of drum revolutions required for image formation decreases the copying speed for making a small number of copies.